This application claims priority of German patent application no. 101 03 100.9, filed Jan. 24, 2001, the complete disclosure of which is hereby incorporated by reference.
The invention relates to a light mixing rod comprising an inlet area and an outlet area, which rod guides light coupled in via the inlet area along a light guiding direction to the outlet area. The invention further relates to the use of such a light mixing rod in an optical device comprising a surface to be illuminated and illumination optics forming an image of the outlet area on the surface to be illuminated.
Such a light mixing rod is employed, for example, for uniformly illuminating an imaging element in a digital projector, as it is called. To this end, light from a light source is coupled into the light mixing rod via the inlet area, so as to generate, at the outlet area, a field illuminating as uniformly as possible and whose image is formed on the imaging element by illumination optics. The image generated by the imaging element is then formed on a projection surface by projection optics.
An important factor for high-quality images in such digital projectors is a defined coupling of the light into the light mixing rod. As the light source, a arc generator, as it is called, is conventionally employed, which comprises a reflector having a primary and a secondary focus, the position of said arc generator ideally coinciding with the primary focus of the reflector, so that the light output in the secondary focus is as high as possible. In order to ensure that the light is coupled in well, the light source and the light mixing rod are arranged such that the secondary focus of the reflector lies in the inlet area. It has been found that both the focal length of the reflector, and thus its secondary focus, and the position of the arc generator in the reflector vary strongly depending on manufacturing conditions, so that the light source requires cumbersome adjustment in order to place the secondary focus in the inlet area.
In view of the above, it is an object of the present invention to improve the aforementioned light mixing rod such that the above-described difficulties may be almost completely overcome.
According to the invention, this object is achieved by a light mixing rod of the aforementioned type in that the light mixing rod comprises two mixing rod portions disposed successively in the light guiding direction and optically coupled with each other, a first one of them being provided as a hollow portion, into which the second mixing rod portion extends at least partially, so that the length of the light mixing rod is easily adjustable. Thus, for adjustment of the length of the entire light mixing rod, the hollow portion may be slid over the second mixing rod portion, until the desired length of the light mixing rod is achieved. In this position, the two mixing rod portions can be connected with each other in such a way that the length of the light mixing rod is fixed. It is further possible to produce a plurality of the two mixing rod portions, each having a constant length, which allows the cost of manufacturing the light mixing rod to be reduced. The length of the individual light mixing rods may then be adjusted individually by pushing the second mixing rod portion into the hollow portion.
Alternatively, the two mixing rod portions may also be displaceable relative to each other in the light guiding direction, so that the length of the light mixing rod is adjustable not only once, but is variable. Thus, in the manner of a telescope, the second mixing rod portion may be introduced into or pulled out of the hollow portion, which comprises reflectively coated internal surfaces, so that the position of the inlet area is also variable, allowing the inlet area to be easily brought into the desired position. This variability of the length of the light mixing rod is particularly advantageous, for example, if such a light mixing rod is installed in a digital projector and the light source of the digital projector needs to be replaced due to a defect. The variability of the length of the light mixing rod then allows the light mixing rod to be optimally adapted to the new light source.
The light mixing rod may preferably be rectilinear or bent.
In a further advantageous embodiment of the light mixing rod according to the invention, the second mixing rod portion is a solid mixing rod portion. Such a solid mixing rod portion is easy to produce and has the advantage that light which does not traverse it directly is guided by total reflexion at the interfaces of the solid mixing rod portions to the air. Since light is reflected without any loss in the case of total reflexion, this solid mixing rod portion only involves the very small losses of material absorption, so that nearly all of the coupled-in light is passed on.
Further, the light mixing rod according to the invention may be particularly embodied such that the end of the hollow portion averted from the second mixing rod portion forms the inlet or outlet area. This advantageously has the effect that the inlet and/or outlet area is not formed by a material interface, but by the cross-sectional area at the end of the hollow portion. Thus, no misting and/or soiling which would adversely affect the optical properties of the light mixing rod can occur on this inlet or outlet area.
A particularly preferred further embodiment of the light mixing rod according to the invention consists in that the light mixing rod comprises a second hollow portion, which is optically coupled with the second mixing rod portion at an end of the second mixing rod portion averted from the first hollow portion. This has the effect that both the inlet area (the end of the second hollow portion averted from the second mixing rod portion) and the outlet area (the end of the first hollow portion averted from the second mixing rod portion) are not provided as material interfaces, so that soiling and/or misting can occur neither on the inlet area nor on the outlet area.
In a further advantageous embodiment of the light mixing rod according to the invention, it comprises a further mixing rod portion, which is optically coupled with the first hollow portion at the end thereof averted from the second mixing rod portion. Preferably, this further mixing rod portion is a solid mixing rod portion. Thus, a light mixing rod is provided wherein the first hollow portion is an optical coupling element which couples the second and the further mixing rod portion with each other. If the second mixing rod portion and the first hollow portion are displaceable relative to each other in the light guiding direction, the length of the light mixing rod is also variable. If the further mixing rod portion is a solid mixing rod portion, this has the advantageous effect that transmission losses in this solid mixing rod portion are extremely small.
Preferably, the light mixing rod may also be provided with an end portion of hollow cross-section and/or with a starting portion of hollow cross-section, with an end of the end portion forming the outlet area and an end of the starting portion forming the inlet area. Thus, a light mixing rod is also provided wherein no soiling and/or misting, which may adversely affect the optical properties of the light mixing rod, can occur on the inlet and outlet areas due to the hollow cross-sections of the end portion and of the starting portion.
Further, the light mixing rod according to the invention may be further embodied, in particular, such that between the two mixing rod portions, which are displaceable relative to each other, a lubricant, such as e.g. a sheet inlay having a predetermined thickness, is provided in their overlapping region. This ensures the displaceability and advantageously also prevents scratching of the reflectively coated internal surfaces of the hollow portion and of the side surfaces of the second mixing rod portion, if the latter is a solid mixing rod portion. The lubricant need not be present in the entire overlapping region, but may also be provided in just one portion thereof.
The light mixing rod according to the invention may be used, in particular, in an optical device comprising a surface to be illuminated and illumination optics which form an image of the outlet area on the surface to be illuminated. Preferably, the optical device also comprises projection optics for projecting the surface to be illuminated onto a projection surface. Thus, an optical device is provided wherein, due to the structure of the light mixing rod, the inlet area may easily be brought into the desired position in the optical device, e.g. relative to a light source, by changing the length of the rod.
The invention will be explained in more detail below by way of example and with reference to the drawings.